1. Field of the Invention
This invention relates to an image read-out apparatus for photoelectrically detecting light carrying image information by use of a photomultiplier.
2. Description of the Prior Art
In general, photomultipliers are widely used in image read-out apparatuses for reading out an image by exposing a recording medium carrying the image stored therein to a light beam and by detecting light reflected by the recording medium, or light transmitted therethrough, or light emitted thereby.
The image read-out apparatus using a photomultiplier is employed, for example, in an input apparatus of a computer and an image read-out apparatus of a facsimile. In the apparatus, image read-out is specifically carried out as described below.
A recording medium carrying an image constituted by a density pattern stored therein, i.e. an original, is two-dimensionally scanned by a light beam. Light reflected by the original (in the case where the original is a paper original) or transmitted therethrough (in the case where the original is a film original) is detected and converted into a serial electric image signal by a photomultiplier. In this manner, the image recorded on the original is read out. The electric image signal thus obtained is subjected to various processings, for example, transfer to a remote station via a transfer system, an image processing, and storing in a magnetic recording medium.
Image read-out using a photomultiplier is conducted also in a radiation image recording and reproducing system using a stimulable phosphor sheet as disclosed, for example, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395. In this case, a stimulable phosphor sheet carrying a radiation image stored therein is exposed to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and the emitted light is detected photoelectrically.
The "stimulable phosphor" referred to in this invention means a phosphor which is able to store radiation energy therein upon exposure to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, and then emit light in proportion to the stored energy of the radiation upon stimulation with stimulating rays such as visible light.
In the radiation image recording and reproducing system using a stimulable phosphor sheet, image read-out is specifically carried out as described below.
A stimulable phosphor sheet carrying a radiation image stored therein by being exposed to a radiation such as X-rays via an object such as the human body is two-dimensionally scanned by stimulating rays such as a laser beam which cause it to emit light in proportion to the stored radiation energy. The emitted light is detected and converted into an electric image signal by a photomultiplier. In this manner, the radiation image is read out. On the basis of the electric image signal, a visible image is reproduced on a recording material such as a photographic film or on a display device such as a cathode ray tube (CRT).
In the image read-out apparatus using a photomultiplier, image read-out is normally conducted by adjusting the read-out gain of the photomultiplier to suit a desired image read-out range. However, when images are read out from recording media carrying various images stored therein, the intensity of light carrying the image information which should be read out is not necessarily within the desired image read-out range, and often goes beyond the desired image read-out range. In such a case, the output current of the photomultiplier for reading out the image does not accurately correspond to the intensity of light incident on the photomultiplier. That is, the output current of the photomultiplier does not change with sufficient sensitivity to small changes in the intensity of light incident on the photomultiplier, and it is not always possible to accurately detect the image.